There is a very modern trend especially among large employers to keep a hard-line discipline on staff and make an example of anyone who raises concerns or shows some independence of thought or makes a very minor mistake. You must be very cautious in your responses to allegations. Take a support person who is ideally independent and from a law firm (it’s not that costly) not the union to any meeting. Do not be rushed into a meeting you should be given at least 48 hours to get your support person of choice. Taking your partner or someone else employed by the company is very foolish. Recording the meeting on your iPhone is very valuable.
The union may be well intentioned, but it is a long time since unions other than the most militant, it is not often that they union is prepared to stand up to an employer for an individual employee. The union may represent the very employees that have raised the issue with the company so there is an obvious conflict of interest which is hard for the union to deal with when it wants to keep everybody happy.
How can they really stand up while at the same time negotiating an EBA or otherwise needing to work with the company to achieve good results for the overall body of employees?
Unions are reluctant to commit to the expense of going to Court and mostly recommend the soft options, for example, resignation as an easy way out. However, employees who resign usually suffer loss of earnings because they have no job to go to and can rarely bring a court case after a resignation. Resignation without a substantial payout and a Deed of Release is rarely a good idea.